Recently, among flat-panel-type display devices, liquid crystal display (LCD) panels are used widely, but they still have problems such as a low response speed, a narrow viewing angle, etc., and most of improved versions of these also have problems such as insufficient properties and high costs of panels. Among these, thin film EL elements have drawn attention as new expected self-luminous light-emitting elements with excellent visibility and high response speeds, which are expected to be applicable in a wide range of fields. Particularly, various studies have been carried out about thin film EL elements having layers all of which, or a part of which, are made of organic materials that can be formed into films by simple film forming processes such as vapor deposition or application at room temperature, which are called organic EL elements. This is because they are attractive due to, in addition to the above-described characteristics, their production costs being reducible to relatively low levels.
A thin film EL element that operates in a direct-current electric field (organic electroluminescent element, hereinafter referred to as “organic EL element” for short) has a light-emitting region present between a pair of electrodes, that is, a hole-injection electrode and a cathode (electron-injection electrode), and achieves light emission by recombination of an electron and a positive hole injected from the foregoing electrodes. Many studies have been made about such an organic EL element, but the emission efficiency thereof generally has been low and they have been far from practical application in a light-emitting element.
Among these, an element proposed by Tang et al. in 1987 (C. W. Tang and S. A. Vanslyke: Applied Physics Letter 51(1987)913 (issue date: Sep. 21, 1987)) was an element having a hole-injection electrode, a hole-transport layer, a light-emitting layer, and a cathode that were provided on a transparent substrate in the stated order, in which indium tin oxide (ITO) was used as the hole-injection electrode, a 75 nm-thick diamine derivative layer was used as the hole-transport layer, and a 60 nm-thick aluminum quinoline complex layer was used as a light-emitting layer, and the cathode was made of a MgAg alloy that has both of the electron-injection capability and the stability against the degradation. In addition to the improvement of the cathode, the use of diamine derivative excellent in transparency for forming the hole-transport layer particularly allowed the hole-transport layer even with a thickness of 75 nm to maintain sufficient transparency, and this thickness made it possible to obtain a uniform thin film without a pin hole or the like. Therefore, this sufficiently reduced a total thickness of an element including the light-emitting layer (to approximately 150 nm), thereby allowing light emission with high luminance to be obtained with a relatively low voltage. More specifically, high luminance of not less than 1000 cd/m2 and high efficiency of not less than 1.51 m/W were obtained with a low voltage of not more than 10 V. This report by Tang et al. initiated active studies for the further improvement of the cathode, and the improvement of the element configuration such as the insertion of an electron-injection layer and the insertion of the hole-injection layer, which have been continued to date.
The following will summarize a thin film EL (organic EL) element that is being studied generally.
To form respective layers of the element, a hole-injection electrode, a hole-transport layer, a light-emitting layer, and a cathode are laminated in the stated order on a transparent substrate. Further, a hole-injection layer may be provided as required between the hole-injection electrode and the hole-transport layer, an electron-transport layer may be provided as required between the light-emitting layer and the cathode, or an electron-injection layer may be provided as required on an interface with the cathode. Thus, by dividing and distributing functions to the respective layers, appropriate materials can be selected for the layers, respectively, thereby improving the characteristics of the element.
Generally, a glass substrate such as “CORNING 1737” (non-alkali borosilicate glass produced by Corning Glass Works) is used widely as the transparent substrate. The substrate preferably has a thickness of approximately 0.7 mm since this makes the substrate easy to handle from the viewpoint of strength and weight.
As the hole-injection electrode, a transparent electrode is used, such as an ITO sputtered film, an ITO electron beam vapor deposition film, or an ITO ion-plating film. The thickness thereof is determined according to a sheet resistance and a visible light transmittance that are required, but in many cases it is set to be not less than 100 nm so as to decrease the sheet resistance, since a driving current density is relatively high in an organic EL element.
For forming the hole-transport layer, a film obtained by vapor deposition of a diamine derivative is used widely, for instance, a diamine derivative used by Tang et al. such as    N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (hereinafter referred to as TPD), or N,N′-bis(α-naphtyl)-N,N′-diphenylbenzidine (hereinafter referred to as NPD). Particularly, a film obtained by vapor deposition of a diamine derivative having a Q1-G-Q2 structure disclosed in U.S. Pat. No. 4,539,507 (issue date: Sep. 3, 1985) (corresponding to JP 2037475 B (JP 59-194399A (publication date: Nov. 5, 1984)) is used widely. It should be noted that each of Q1 and Q2 is a group containing a nitrogen atom and at least three carbocyclic rings (at least one of which is aromatic), and G is a linking group composed of a cycloalkylene group, an arylene group, an alkylene group, or a carbon-to carbon bond. These materials generally have excellent transparency such that they are substantially transparent even if it is formed into a film having a thickness of approximately 80 nm, and have excellent film forming properties. Therefore, by using the same, it is possible to form a film without defects such as pin holes, and problems relating to the reliability such as short-circuit hardly occur, even if a total thickness of the element is reduced to approximately 100 nm.
The light-emitting layer also is, as reported by Tang et al., formed by vapor deposition of an electron-transport light-emitting material such as tris(8-quinolinolato)aluminum so as to have a thickness of several tens of nanometers. For achieving emitted lights of various colors, the light-emitting layer is a relatively thin film, and in some cases, a so-called double hetero structure having an electron-transport layer with a thickness of approximately 20 nm is used.
In many cases, the cathode used is a cathode made of an alloy such as a MgAg alloy or an AlLi alloy proposed by Tang et al., or a layered cathode. The alloy forming the cathode is an alloy of a metal that has a low work function and a low electron-injection barrier and a metal that has a relatively high work function and is stable. The layered cathode is formed by laminating, for instance, an electron-injection layer of various types such as LiF and aluminum.
Further, in addition to such a lamination configuration of “hole-transport layer/electron-transport light-emitting layer”, the configuration of “hole-transport light-emitting layer/electron-transport layer”, and the configuration of “hole-transport layer/light-emitting layer/electron-transport layer” are used widely. With use of any one of the lamination configuration, the same transparent substrate, hole-injection electrode, and cathode as those described above are used in the same manner.
Generally, it is almost impossible to obtain organic compounds that have excellent electron transport capability, and relatively limited compounds can be used for forming the lamination configuration of “hole-transport layer/electron-transport light-emitting layer”. In contrast, in the case of the configurations of “hole-transport light-emitting layer/electron-transport layer” and “hole-transport layer/light-emitting layer/electron-transport layer”, various types of materials can be used for forming light-emitting layers. Therefore, they have possibilities for providing various colors of light and high performance in the efficiency and the lifetime, and hold high expectations.
For instance, U.S. Pat. No. 5,085,947 (issue date: Feb. 4, 1992) [corresponding to JP 2-250292A, date of publication: Oct. 8, 1990] discloses an element with a configuration of “hole-transport light-emitting layer/electron-transport layer” in which    [4-{2-(naphthalen-1-yl)vinyl}phenyl]bis(4-methoxyphenyl)amine, or    [4-(2,2-diphenylvinyl)phenyl]bis(4-methoxyphenyl)amine is used as a hole-transport light-emitting material, and an oxadiazole derivative is used for forming an electron-transport layer.
Further, WO 96/22273 (international publication date: Jul. 25, 1996) discloses an organic thin film EL element having a configuration of “hole-transport layer/light-emitting layer/electron-transport layer” in which 4,4′-bis(2,2-diphenyl-1-vinyl)-1,1′-biphenyl, which is a hole-transport light-emitting material, is used for forming a light-emitting layer.
Still further, in the Spring Annual Session G2.1 Lecture of Material Research Society (MRS) in 1998 (oral presentation, Apr. 13, 1994), an element was reported that has a configuration of “hole-injection layer/hole-transport light-emitting layer/hole blocking layer/electron-transport layer” in which NPD, which is a compound of the Q1-G-Q2 type proposed by Tang et al., is used as a hole-transport light-emitting material.
Thus, the use of not only an electron-transport light-emitting material but also a hole-transport light-emitting material as a light-emitting material enables the designing with a wide-range of materials, and allows light emission with various colors to be obtained. However, it still has not been possible to obtain those with sufficient characteristics regarding the emission efficiency and lifetime. In particular, it is said that in the case where a fluorescent material is used, only 25% of an excited state generated by recombination of an electron and a hole contributes to light emission, and this has been a significant problem in pursuing a further improved efficiency.
In such a situation, recently, many studies have been done about an element in which a light-emitting layer obtained by doping a host material with a heavy metal complex, as disclosed in Applied Physics Letter, vol. 75, No. 1, pages 4 to 6 (issue date: Jul. 5, 1999). It is reported that in such an element, due to a heavy metal effect, a triplet exciton that is said to inherently make a forbidden transition and therefore does not contribute to light emission is caused to make a luminescent transition to a ground state, thereby allowing the triplet exciton, which is said to be generated at a rate of 75%, to be used in light emission. Therefore, the foregoing element can achieve a higher efficiency.
However, fac tris(2-phenylpyridine)iridium [abbreviated as “Ir(ppy)3”] disclosed in the foregoing thesis and many other heavy metal complexes cannot necessarily be synthesized or purified easily. On the other hand, a layer made of a single material does not have a sufficient charge transport capability, and exhibits significant concentration quenching (the decrease of the emission intensity at or above a certain concentration). Therefore, it has been usual to use a charge transport host material doped with a heavy metal complex at an appropriate concentration, but the efficiency and the lifetime depend on the dopant concentration, thereby causing disadvantages in the production.
Considering these conditions, the inventors of the present invention not only designed heavy metal complex materials of various structures and studied characteristics thereof in detail, but also studied a wide range of light-emitting elements in each of which a light-emitting layer contained a mixture of a heavy metal and a compound that were selected from a variety of the same and that were deposited independently from each other. As a result, the inventors found the following. A heavy metal and a compound for contributing to light emission need not be one compound as a complex. Even in the case where a compound for contributing to light emission and a heavy metal physically were mixed and present close to each other, it was observed widely that the emission efficiency was improved significantly as compared with the case where no heavy metal was contained. With this, the present invention was completed. Further, the inventors also found that the emission efficiency further improved in the case where the heavy metal mixed therein was in as fine a state as possible. Thus, the present invention was completed.